2.4.3 Configuration events

When the value of a configuration parameter is altered, either
directly or by loading a configuration file, a configuration
event is generated. Handlers can be registered to be notified when a
configuration event occurs. A configuration event is associated with
the group and key that had its value modified.

If a configuration event is emitted by a configuration context, it
propagates to all configuration contexts which inherit that group and
key from it.

A configuration event handler must be a closure that accepts three
arguments:

handler cfg group key

cfg is always the configuration context that received the event,
and the group and key identify the configuration parameter
that changed.

— Function: add-config-event! cfg handler

Registers handler to be called when configuration changes in the
context cfg.

— Function: remove-config-event! cfg handler

Stops handler from being called when configuration changes in
the context cfg.